Returns a localised string describing an decoder for the format specified in
caps, for use in error dialogs or other messages to
be seen by the user.
Should never return NULL unless
caps are invalid.
This function is mainly for internal use, applications would typically use missing_plugin_message_get_description to get a description of a missing feature from a missing-plugin message.
the (fixed) Caps for which an decoder description is needed
a newly-allocated description string, or NULL on error. Free string with g_free when not needed any longer.